It depends of how helpful they are, if I have to put my own bags in and out...$0 if they pick up my bags at the airport are on time have nice attitude up to $20 (because of my itinerary, I didn't needed all my bags with me in Cartagena, during my flight connection in Bogota the staff from the Marriott Bogota picked the lugagge up from me at the airport and held it for a couple of days while I got to the hotel a couple of days later). I think that the service you receive should be a direct correlation to the tip.
I think the whole tip situation in the United States at least has gotten out of control. There are tip jars everywhere including when I picked up my shirts this week at the cleaners and low and behold there was a tip jar. I certainly can see in restaurants since they don't pay them and tips become the major portion of their income or when someone gives you special service but now it seems that any employee that comes in contact with a customer ever so incidental is expecting a tip . Just my view
never used it; too expensive;
when I have the chance/choice I always give the same taxi drivers in Beijing and Shanghai a call;
I don't have many bags, I take care of them myself, I am always on time
and they appreciate the business
only when my flight is delayed and they have to wait for me, then I give them a tip of around 100RMB (12 dollar)